[Greenbuilding] [BULK] Re: vented roofs

[George J. Nesbitt]

You made a good choice, although conservative.
    Vented attics work in all climates as long as you have a tight air 
barrier on the ceiling, unvented attics in all except hot dry climates 
should have foam to the outside (according to Joe Lstiburek, a great 
Canadian EH!). Although I agree with Robert W. Tom that you can probably 
get away with Dense Pack Cellulose (3.5lbs cuft or greater) in an 
unvented roof, although I think thats more true in a cold northern 
climate more than a warm moist southern climate.
    According to J.D. Ned Nisson in The Supperinsulated House book if 
you use foam on the outside of assemblies 2/3 of the insulation should 
be on the outside. The last time I saw Joe Lstiburek he was heavily 
suggesting that if you put a vapor barrier in an assembly, that the 
inside surface never reach dew point, so you don't get condensation and 
moisture problems.
    I am partial to vented attics for many reasons, they work better 
than the equivalent roof ceiling, you can get up into them to see whats 
going on, you can add/change wiring and other services. At my house I 
plan to insulate with 4" polyisocyanerate on the exterior of my roof and 
bring my attic into conditioned space or at least an unconditioned 
space. I am going to place my hydronic air handler and ducts in the attic.


Lawrence Lile wrote:

> 
>I think the conspiracy theory holds some water.  The thing that the
>fiberglass companies are embarrassed to say is, their insulation wimps
>out at low temperatures - the R value actually drops when the
>thermometer drops.
>
>I can speak to the vented cathedral ceiling idea. It is not difficult to
>do at all.  I've got a cathedral ceiling made out of 24" trusses, and
>plan on using cellulose.  Despite some strong arguments on this list, I
>could not be convinced to install a "hot roof" nonventilated attic.  I
>still don't know if I made the right choice, but that's how we are
>building it.  
>
>The insulation contractor plans to install insulation screen, then blow
>cellulose above that.  It won't be dense packed, so it will settle some,
>but we should be able to pack 18" in there easily which will be plenty
>after settling. Gyp board goes on when they are done.  No vapor barrier
>on the interior. I have been very careful to avoid any penetrations in
>the ceiling gyp board, no ceiling can lights and such, and have been
>caulking up little things like electrical penetrations in the top plate.
>There should be very little air movement to transport bulk moisure into
>the attic. 
>
>We could have done the same thing with a 16" truss and a hot roof
>design. Either way, I don't think it is hard to do.   
> 
>
>--Lawrence Lile, PE, LEED AP
>Project Solutions Engineering www.projsolco.com
>
>
>
>-----Original Message-----
>From: greenbuilding-bounces at listserv.repp.org
>[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of Laren
>Corie
>Sent: Saturday, September 16, 2006 2:21 PM
>To: greenbuilding at listserv.repp.org
>Subject: [BULK] Re: [Greenbuilding] vented roofs
>Importance: Low
>
>"Janet L. Hobbs" <janethobbs at hobbsink.com>
>
>  
>
>> I have heard but not verified that some shingle manufacturers won't 
>>warranty their product if it isn't ventilated.
>>    
>>
>
>Hi Janet;
>
>  That is true;
>
> They say that heat is the reason for the invalidation of the guarantee
>However, it is very interesting that testing shows that darker shingles,
>over vented attics, consistently run much hotter than lighter colored
>shingles with no venting.  That reason is not logical.
>
>So, there must be more at play here, than is first apparent.
>
>1) The big shingle companies that set the standards, are also the major
>fiberglass insulation manufacturers.
>2) The unvented roof insulations are the same insulations (cellulose and
>foams) that are major competition to fiberglass.
>(if 8-9%, in the US, can really be called major competition)
>3) The fiberglass industry has an extremely well organized program to
>dis-educate the public against alternatives.  Just Google "cellulose
>insulation" then look whose pages come up, and how most of them use
>identical extremely well written, but even more deceptive, text from
>their insulation manufac- -turers association, that only allows
>fiberglass manufacturers.
>
>   The limit on the warrantee is a very clever way to get two advantages
>at once  They can both get out of their guarantee, while screwing up a
>good sale for their insulation competition.
>Cellulose is, by far, the main competition. Vented cathedral ceilings
>with cellulose are generally too much bother to be cost effective.
>
>That seems to be the only understanding that makes full sense.
>If anyone has contrary information, please present it.  I would hate to
>be seeing evil, where it does not exist, but nothing else seems to be
>logical, in this case.  Their fiberglass industries'
>cellulose websites are definitely in a category that I consider
>immorally deceptive.  They have some very good writers, who could
>convince you to stop breathing to avoid second hand smoke in the middle
>of Antarctica,
>
>-Laren Corie-
>Natural Solar Building Design Since 1975 www.LarenCorie.com
>
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